ensure cleanup isn't called multiple times

Stephen von Takach · Stephen von Takach · commit 5f717f1c0e96 · 2017-10-16T11:23:34.000+11:00
fixes #11
diff --git a/lib/ruby-tls/ssl.rb b/lib/ruby-tls/ssl.rb
@@ -689,6 +689,7 @@ def negotiated
 
             SSL_RECEIVED_SHUTDOWN = 2
             def cleanup
+                return unless @ready
                 @ready = false
 
                 InstanceLookup.delete @ssl.address
